1. Field of the Invention
The present invention relates to a process for producing a dense anhydrous sodium carbonate crystal by an aqueous solution reaction.
2. Description of the Prior Arts
The practical process employed as an industrial process for producing a dense anhydrous sodium carbonate crystal is characterized by calcining sodium bicarbonate to form porous anhydrous sodium carbonate; hydrating it into sodium carbonate hydrate; and treating it by a steam tubed dryer (STD). This process is complicated and requires a large energy consumption. In this process, a dense anhydrous sodium carbonate crystal is produced, but is not completely dense because the spaces are remained by evaporating water in the dehydration in the steam tubed dryer. The bulk density of the product has been in a range of about 1.0 to 1.1.
On the other hand, a process for producing the dense anhydrous sodium carbonate in an aqueous solution reaction has been reported.
On the process disclosed in U.S. Pat. No. 2,133,455, wherein sodium hydroxide or sodium chloride are added to an aqueous solution obtained by decomposing sodium bicarbonate to decrease the transition temperature between sodium carbonate monohydrate and anhydrous sodium carbonate to be lower than a boiling point of the solution, and the solution is concentrated in an evaporator to crystallize an anhydrous sodium carbonate crystal. The anhydrous sodium carbonate crystal is crystallized by this process. In this process, the solution is concentrated by evaporating water by an evaporation whereby the energy consumption is remarkably large. In order to reduce the transition temperature to a desired temperature, it is necessary to incorporate 10 wt. % of sodium chloride or more. The incorporated sodium chloride is disadvantageously included as the impurity in high level in the anhydrous sodium carbonate crystal. When the amount of sodium hydroxide is too much, the growth of the crystal is inhibited or the viscosity of the solution is increased to cause diadvantageously troubles in the operation.
The other known process is the process disclosed in Japanese Unexamined Patent Publication No. 7530/1980, wherein anhydrous sodium carbonate is produced by the transition of sodium carbonate hydrate in an aqueous solution of sodium carbonate containing 10 to 22 wt. % of sodium chloride at a temperature of 3.degree. to 7.degree. C. higher than the transition temperature for the transition. In this process, the content of sodium chloride is large to contaminate the product with a relatively large amount of sodium chloride as the impurity.